Same problem with an iPhone 6 battery that needed replacing, booked in to a Premium retailer in Durham, arrived 30 minutes early and was told they can't see me until the time booked. Waited in a coffee shop nearby and no one entered the shop so they had no previous bookings. Went in at the booked time and had to wait for 10 minutes!!, they took one look at the iPhone and said they wanted 260 pounds to change the battery and it would take a week (suspect they send it back to an Apple Store). On the Apple site it states 75 pounds so I walked out and booked a Genius Bar appointment at Gateshead Metrocentre a week later and they change the battery in an hour. Unfortunately they caught a cable and the FaceTime camera stopped working, outcome, they swapped a 2 year old iPhone 6 out for what looked like a brand new one on the outside with a new battery, not bad for 75 pounds.
Moral of story, always go to an Apple Store even if it takes a week or two and steer clear of their Premium retailers.
CliveS